Added standard bundle installation instructions

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | -
| Fixed tickets | #4157

Note: if you think that this draft is awful, please say it so and we'll rewrite it from scratch.
but ``..raw`` does not achieve this. It let you pass unprocessed context (i.e. either ``html`` or ``latex`` or other format, which is why it asks you for a format btw: you have to provide it for each output format).
What you expect in your image is still a ``<pre>`` tag in HTML, and it is technically a code-block (so ``<pre><code>`` is more semantic).
the only difference with the rendering of ``..code-block`` is that you don't want line numbers (this is fine, Sphinx has an option to omit them in its directive) and a different background color (and this is a matter of theming) But I could totally live with the code being on a block background, and people will be able to copy-paste it.
See http://symfony.com/doc/current/contributing/code/patches.html#make-a-pull-request and you will see that the code snippet containing the pull-request template can totally be copy-pasted (we did not even bothered about removing line numbers on it)